Hardware Interface Description - KORE Telematics

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XT55/56 Hardware Interface Description Confidential / Released s Figure 30 above shows the block diagram of the XT55/56 GPS receiver architecture. The GPS module can be separated into four major parts: RF frequency down-converter, digital baseband demodulation, embedded ARM microprocessor and internal GPS software stored on-board (1 MByte) Flash-Memory. The RF frequency conversion and the baseband demodulation are executed by hardware while the embedded ARM processor computes the GPS position, velocity and time solution employing the internal GPS software. The purpose of the RF circuitry is to reinforce the very weak (-130dBm nominal) GPS signal, to filter it and to down-convert it to an Intermediate Frequency (IF) of 9.45MHz for digital processing. The SiRFstarII architecture relies on the high level of integration in the GRF2i to significantly reduce part count and circuit complexity. A IF filter is built-in as well. • The digital baseband demodulator takes the quantified GPS signal and detects the individual satellites serial data bit stream, along with the associated pseudo range. This action consists of removing spread spectrum and Doppler frequency components of the signal to obtain the serial data messages. • The embedded ARM processor monitors channel allocation, extracts the raw satellite tracking data, computes the position and time solution and sends it on a serial port for high level applications to use or processes it locally. Support functions for the microprocessor include real time clock and reset pulse generator circuits. The internal GPS software monitors and allocates channels, computers the position, velocity and time using the pseudo-range of the satellites and reformats the data to be output or used locally. The internal GPS software is a tasking based architecture driven by the 100ms interrupt generated by GPS2e internal hardware. 4.9 Operation procedure When the receiver is powered up, it steps trough a sequence of states until it can initially determine position, velocity and time. Afterwards, the satellite signals are tracked continuously and the position is calculated periodically. In order to perform a navigation solution (3D solution), the receiver needs. • Pseudo-ranges for at least 3 satellites • Ephemeris data for the satellites it will use in the navigation solution. Note: If almanac navigation is enabled, the receiver can calculate a position without downloading ephemeris data (with a significant position error compared to an ephemeris based solution). The initial position calculation is made, using a Least-Squares Algorithm. Successive position calculations are performed with a Kalman Filter. To generate a position (3D solution) calculation the receiver needs at least 4 measurements to different satellites. In order to calculate a position (Latitude/Longitude/Height), as a 2D solution with an estimated height value, then 3 different satellites are required. Pseudo-range and carrier phase information are available to the position determination algorithms if the receiver has found a SV (acquisition) and can track the signal thereafter. Ephemeris data for a SV can be decoded from orbit data once the GPS signal has been acquired. Each SV transmits its own ephemeris data, the broadcast lasts for 18 seconds, repeating every 30 seconds. The receiver stores ephemeris data in battery-backed memory. This data can be used in future startup’s to improve the time to first fix (TTFF). The ephemeris can also be supplied to the receiver via the serial port. XT55/56_hd_v02.06a Page 78 of 125 17.12.2004
XT55/56 Hardware Interface Description Confidential / Released s 4.10 Start-up procedures The start-up strategy of the XT55/56 GPS part depends on the last position, current time and ephemeris data, that the receiver has stored the external SRAM memory. There are three different start-up procedures: 4.10.1 Coldstart The coldstart takes place when the receiver has no knowledge of its last position or time. In this case the GPS RTC has not been running and no valid ephemeris data or almanac data is available (The receiver has never been navigating or no battery backup memory available). 4.10.2 Warmstart This start-up procedure is performed whenever the receiver is able to use the valid almanac data, and has not in an important manner moved since the last valid position calculation. This procedure starts if the receiver has been shut off for more than 2 hours, but the last position, time and almanac are still acknowledged. This procedure is able to announce the current visible satellites in advance. However, since ephemeris data are not available or cannot longer be used, the receiver has to wait for the ephemeris broadcast to complete. 4.10.3 Hotstart Hotstart is performed whenever the receiver still has access to valid ephemeris data and exact time. This procedure starts if the receiver has been shut off for less than 2 hours and the GPS RTC has been running during that time. Furthermore, during the previous session, the receiver must have been navigating (to allow it to decode and store ephemeris data). In Hotstart, the receiver can announce in advance the currently visible satellites, and is therefore able to quickly obtain and track the signal. Due to the fact that ephemeris is already known, there is no need to wait for the ephemeris broadcast to complete. XT55/56_hd_v02.06a Page 79 of 125 17.12.2004
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